Four of Uber's 10 biggest cities are now in China, according to an email sent by Uber CEO Travis Kalanick to investors and obtained by the Financial Times this summer.

Uber has been investing heavily in China, and the service is growing there like crazy. Uber's service is taking off in China much faster than it did in the United States. Nine months after launching in Chengdu, Uber had 479 times the trips it had in New York after the same amount of time.

Uber is also putting a lot of money into its Chinese growth. Uber's China branch has closed a funding round that values it at $7 billion. In total, Uber has raised more than $6 billion in several funding rounds, including a $600 million investment from Chinese search engine company Baidu.

"The company is selling the future in Asia," this person said. "I don't even think they're talking about self-driving cars. They're saying, if 30% of our revenue is in Asia, imagine how our company will look in the next five years if we nail India and China. Look at the potential market waiting for you."

In September, Didi Kuaidi — Uber's biggest competitor in China — and Lyft, Uber's biggest US rival, announced the two companies were teaming up to take on Uber.

The strategic partnership will let the companies share technology, product development, and local resources, and when US users of Lyft go to China (or when Didi users come visit the US), they'll be able to pay in their native currency on each app.

Now that Lyft has teamed up with Didi, the two companies seem better suited to take on Uber. Didi recently closed a $3 billion round of venture capital funding, and it's valued at $15 billion, so it's well-equipped financially — in total, Didi Kuaidi has amassed a $4 billion war chest of venture capital funding.